This invention relates to a mobile station and weighting control method. More particularly, the invention relates to a mobile station, and to a method of controlling weighting of the mobile station, in a communication system for transmitting the same data from a plurality of base stations to a mobile station on a first channel and transmitting other data from one base station to the mobile station intermittently on a second channel during communication on the first channel.
Closed-Loop Transmit Diversity System
In a closed-loop transmit diversity system, a radio base station of a cellular mobile communication system is provided with a plurality of an antenna elements. The system (1) subjects a plurality of identical transmit data signals to different amplitude and phase control based upon feedback information that is sent from a mobile station, (2) multiplexes pilot signals onto the transmit data that has undergone the amplitude and phase control and transmits the resultant signals using different antennas, and (3) again decides the feedback information (amplitude and amount of phase control) using downlink pilot signals, multiplexes the information onto an uplink channel signal and transmits the resultant signal to the side of the base station. The above-described operation is thenceforth repeated.
With closed-loop transmit diversity in W-CDMA, which is a third-generation mobile communication system, a scheme that uses two transmit antennas is employed, as illustrated in FIG. 9. In FIG. 9, mutually orthogonal pilot patterns P1, P2 are generated in a pilot signal generator 11, the signals are incorporated into transmit data in combiners CB1, CB2 and transmitted from transmit antennas 10-1, 10-2, respectively. A channel estimation unit (not shown) on the receiving side of a mobile station correlates the receive pilots signals and corresponding known pilot patterns, whereby it is possible to estimate channel-impulse response vectors h1, h2 from the transmit antennas 10-1, 10-2 of the base station to a receive antenna 12 at the mobile station.
A weighting calculation unit 13 uses these channel estimation values to calculate an amplitude and phase control vector (weight vector) w=[w1, w2)T of the transmit antennas 10-1, 10-2 of the base station, which vector maximizes power P indicated by Equation (1) below. The vector is quantized, multiplexed onto the uplink channel signal as feedback information and transmitted to the side of the base station. It should be noted that it is unnecessary to transmit both values w1, w2, it being sufficient to transmit only the value w2 in a case where w1 is obtained as w1=1.P=wHHHHw  (1)H=[h1,h2]  (2)
Here h1, h2, represent the channel-impulse response vectors from the transmit antennas 10-1 and 10-2, respectively. Further, the suffix H at the upper right of HH and wH indicates taking the Hermitian conjugate of H and w.
The mobile station calculates the weighting coefficients (weight vector) in the weighting calculation unit 13, multiplexes the weighting coefficient onto the uplink transmit data as feedback information FBI using a multiplexer 18 and transmits the information to the base station from a transmit antenna 14.
At the base station the feedback information from the mobile station is received by a receive antenna 15, the weighting coefficients w1, w2, which are the control quantities, are extracted by a feedback information extraction unit 16, and an amplitude and phase controller 17 multiplies the downlink transmit data by the weighting coefficients w1, w2 using multipliers MP1, MP2 and controls the amplitude and phase of the signals transmitted from the transmit antennas 10-1, 10-2. As a result, the mobile station is capable of receiving the signals transmitted from the two diversity transmit antennas 10-1, 10-2 in an efficient manner.
Feedback Information FBI
Two methods are stipulated in W-CDMA, namely a mode 1, in which the weighting coefficient w2 is quantized to one bit, and a mode 2, in which the weighting coefficient w2 is quantized to four bits. Mode 1 is a method of exercising control in such a manner that the phases of receive signals from each of the transmit antennas will be made approximately the same phase at a resolution of π/4. In mode 1, 1-bit feedback information is transmitted every slot. Control is performed upon finding a phase control amount from two slots of information (an even slot indicates either 0 or π by a single bit, an odd slot indicates either π/2 or 3π/2 by a single bit, and the information is the average of these). As a result, control speed is high but accurate control cannot be performed because quantization is coarse. In mode 2, however, control is performed in such a manner that the phases of receive signals from each of the transmit antennas is made approximately the same phase at a resolution of π/4, and the ratio of transmission power of the transmission signal from each transmit antenna is controlled, with 4-bit information being used to achieve this control. As a result, highly accurate control can be achieved. However, one bit at a time is transmitted in each slot and one word of feedback information is transmitted in four slots. If the fading frequency is high, therefore, follow-up will not be possible and a degraded characteristic will result.
FIG. 10 is a diagram showing the structure of an uplink DPCH (Dedicated Physical Channel) frame standardized by the 3rd Generation Partnership Project (referred to as “3GPP” below). A DPDCH (Dedicated Physical Data Channel) on which only transmit data is transmitted and a DPCCH (Dedicated Physical Control Channel) on which a pilot and control data such as feedback information are multiplexed and transmitted are multiplexed by orthogonal codes. More specifically, in the frame format of an uplink signal from a mobile station to a base station, one frame has a duration of 10 ms and is composed of 15 slots (slot #0 to slot #14). The DPDCH is mapped to an orthogonal I channel of QPSK modulation and the DPCCH is mapped to an orthogonal Q channel of QPSK modulation. Each slot of the DPDCH consists of n bits, and n varies in accordance with the symbol rate. Each slot of the DPCCH consists of ten bits, has a symbol rate of a constant 15 ksps and transmits a pilot PILOT, transmission power control data TPC, a transport format combination indicator TFCI and feedback information FBI. The PILOT is utilized on the receiving side to perform channel estimation (estimation of propagation path characteristics) and when measuring SIR. The TFCI transmits the symbol speed of data and the number of bits per frame, etc. The FBI transmits the above-mentioned feedback information (weighting coefficients) for controlling the transmit diversity at the base station.
Structure of Radio Mobile Station
FIG. 11 illustrates an example of the structure of a radio mobile station. A downlink data signal from the base station is received by the receive antenna 12 and sent to a data channel despreader 20 and pilot channel despreader 22. The data channel is despread by the data channel despreader 20 and the pilot channel by the pilot channel despreader 22. Despread pilot signals P1′, P2′, which are the result of processing by the pilot channel despreader 22, are input to channel estimation units 23-1, 23-2 and to the weighting calculation unit 13.
The channel estimation units 23-1, 23-2 compare the receive pilot signals P1′, P2′ and known pilot signals P1, P2 in order to obtain the channel estimation values from the transmit antennas 10-1, 10-2 of the base station to the receive antenna 12. The channel estimation units 23-1, 23-2 obtain channel impulse responses h1, h2, which indicate the state of amplitude and phase modulation ascribable to propagation of the receive pilot signals and input these responses to a receiving unit 21. The latter applies channel compensation processing to the data-channel signal and inputs the result to a demodulator and decoder, not shown.
The weighting calculation unit 13 finds weighting coefficients w1, w2 that will maximize the power P indicated by Equation (1) and outputs the feedback information FBI. That is, the weighting calculation unit 13 has a phase/amplitude comparator 13a for comparing the phases and the amplitudes of the pilot signals P1′, P2′ received from the transmit antennas 10-1, 10-2; and an FBI generator 13b for generating the feedback information FBI conforming to the weighting coefficients w1, w2 and inputting the information to the multiplexer 18. The latter multiplexes the feedback information and transmit data signal. A data modulator 25 performs orthogonal modulation based upon the multiplexed data, and a spread-spectrum modulator 26 applies spread-spectrum modulation and transmits an uplink data signal, which contains the feedback information, from the transmit antenna 14 toward the base station.
Handover
FIG. 12 illustrates an example of the structure of a conventional system at the time of handover. This illustrates an example of a case where handover is performed between two base stations 1 and 2. Components identical with those shown in FIG. 9 are designated by like reference characters. It should be noted that all antennas of base stations 1,2 and mobile station are used for both sending and receiving. Further, the feedback information extraction unit 16 and amplitude and phase controller 17 of FIG. 9 have been consolidated and are additionally provided with an antenna assigning function and illustrated as antenna assigning/weighting controllers 19, 19′. The base stations 1, 2 are identically constructed. Handover is carried out by sending and receiving messages in a higher-order layer between the base stations 1, 2, a base-station control unit 3, which serves as a host device, and the mobile station 4. The base stations 1 and 2 are each provided with two transceive antennas 10-1, 10-2 and 10-1′, 10-2′, respectively.
Before handover, the mobile station 4 receives the pilot signals P1, P2 currently being transmitted and calculates optimum weights w1, w2 of transmit diversity. Further, when the soft handover state is attained, the mobile station 4 receives signals from both base stations 1, 2 simultaneously, combines and outputs the diversities and calculates a control vector w that will maximize the following equation:P=wH(H1HH1+H2HH2)w  (3)where Hk is the channel impulse response of the signal from a kth base station, H1 can be estimated by the pilot signals P1, P2, and H2 can be estimated by pilot signals P3, P4. After a changeover has been made to base station 2 by handover, antenna weights W3, W4 are calculated using the pilot signals P3, P4 of the base station 2 at the destination of handover.
In closed-loop transmit diversity in W-CDMA, use is made of HSDPA (High-Speed Downlink Packet Access), which is capable of high-speed data transmission in the downlink direction (see References 1, 2).
Reference 1: 3G TS 25.212 [3rd Generation Partnership Project: Technical Specification Group Radio Access Network; Multiplexing and channel coding (FDD)]
Reference 2: 3G TS 25.214 [3rd Generation Partnership Project: Technical Specification Group Radio Access Network; Multiplexing and channel coding (FDD)]
HSDPA will be described in brief below.
HSDPA
HSDPA is a method of adaptively controlling transmission rate in accordance with the radio environment between a radio base station and a mobile station and performs retransmission control H-ARQ (Hybrid Automatic Repeat reQuest) based upon receive success/failure. The principal radio channels used in HSDPA are (1) HS-SCCH (High Speed-Shared Control Channel), (2) HS-PDSCH (High Speed-Physical Downlink Shared Channel) and (3) HS-DPCCH (High SpeedDedicated Physical Control Channel), as illustrated in FIG. 13.
HS-SCCH and HS-PDSCH are both shared channels in the downlink direction (namely from the radio base station to the mobile station). HS-SCCH is a control channel that transmits various parameters relating to data transmitted on HS-PDSCH. In other words, it is a channel for giving notification of the fact that transmission of data is carried out via HS-PDSCH. Examples of the various parameters are the following items of information: destination information indicating to which mobile station data is to be transmitted, modulation scheme information indicating what modulation scheme is to be used to transmit data by HS-PDSCH, and information such as pattern of rate matching performed with respect to transmit data.
HS-DPCCH is a dedicated control channel in the uplink direction (namely the direction from the mobile station to the radio base station) and is used in a case where result (an ACK signal or NACK signal) of reception is transmitted to a radio base station in accordance with whether or not there is an error in data received by a mobile station via the HS-PDSCH. That is, HS-DPCCH is a channel used in order to transmit the result of reception of data received via the HS-PDSCH. If a mobile station has failed to receive data (if the receive data is a CRC error, etc.), the NACK signal is transmitted from the mobile station and therefore the radio base station executes retransmission. In addition, the HS-DPCCH is used in order that a mobile station, which has measured the reception quality (e.g., the SIR) of a signal received from a radio base station, may transmit this reception quality to the base station as a CQI (Channel Quality Indicator). That is, the CQI is information whereby the mobile station reports the reception environment to the base station. The CQI takes on values of 1 to 30. A CQI for which block error rate BLER does not exceed 0.1 in this reception environment is reported to the base station.
The radio base station determines whether the radio environment in the downlink direction is good or not based upon the received CQI. If the environment is good, a changeover is made to a modulation scheme whereby data can be transmitted at higher speed. Conversely, if the environment is no good, then a changeover is made to a modulation scheme in which data is transmitted at lower speed. (In other words, adaptive modulation is carried out.) In actuality, the base station holds a CQI table that defines formats of different transmission rates in accordance with CQIs of 1 to 30. The parameter (transmission rate, modulation scheme, number of multiplex codes, etc) conforming to the CQI is found from the CQI table and the data is transmitted to the mobile station on HS-PDSCH based upon the parameter.
Channel Structure
FIG. 14 is a diagram useful in describing channel timing in an HSDPA system. Since code division multiplexing is employed in W-CDMA, the channels are separated by codes. CPICH (Common Pilot Channel) and SCH (Synchronization Channel) are shared channels in the downlink direction. CPICH is a channel utilized in channel estimation and cell search, etc., at a mobile station, and is for transmitting a so-called pilot signal. Strictly speaking, the SCH includes a P-SCH (Primary SCH) and an S-SCH (Secondary SCH). These are channels on which a signal is transmitted in burst fashion by 256 chips at the beginning of each slot. The SCH is received by a mobile station that performs a three-stage cell search and is used to establish slot synchronization and frame synchronization and to identify the base-station code (scramble code). Although SCH has a length that is 1/10 of a slot, it is illustrated as having a larger width in FIG. 14. The other 9/10 of the slot is a P-CCPCH (Primary-Common Control Physical Channel).
The timing relationship of the channels will be described next. Each channel constructs one frame (10 ms) from 15 slots, and one frame has a length equivalent to 2560 chip lengths. Since the CPICH is used as a reference for other frames, as mentioned earlier, the leading ends of the frames of SCH and HS-SCCH coincide with the leading end of the frame of CPICH. On the other hand, the leading end of the frame of HS-PDSCH lags behind HS-SCCH, etc., by two slots. The reason for this is to make it possible for the mobile station to perform demodulation of HS-PDSCH by a demodulation scheme that corresponds to the modulation scheme after the information of this modulation scheme is received via the HS-SCCH. Further, HS-SCCH and HS-PDSCH each construct one subframe from three slots.
HS-DPCCH is a channel in the uplink direction. A first slot thereof is used to transmit the ACK/NACK signal, which indicates the result of reception of HS-PDSCH, from the mobile station to the radio base station upon elapse of approximately 7.5 slots following reception of HS-PDSCH. Second and third slots are used to feed back the CQI information for adaptive modulation control to the base station periodically. The CQI information transmitted is calculated based upon the reception environment (e.g., the result of measuring the SIR of CPICH) measured in an interval of the CQI transmission from four slots earlier to one slot earlier.
Handover at Time of Communication by HS-PDSCH and DPCH
If the mobile station 4 is handed over [see (B) of FIG. 15] owing to movement thereof when the mobile station is performing voice communication only on DPCH [see (A) of FIG. 15], the same voice data is sent from on DPCH from both base stations 1 and 2. The mobile station 4 therefore handles the receive signals from both base stations equivalently, calculates the weighting coefficient w in accordance with Equation (3) and feeds the signal back to each base station. Further, since the same voice data is sent on DPCH from both base stations, the mobile station diversity-combines the signals received from both base stations, outputs the result and produces diversity gain.
With W-CDMA that employs the HSDPA scheme, a case arises where communication by the HS-PDSCH is performed at the same time as communication by the DPCH in such a manner that the user may perform voice communication while browsing a website. In such simultaneous communication, if data (a packet) from the Internet is to undergo high-speed transmission, for example, the data is transmitted from base station 1 to mobile station 4 at high speed on HS-PDSCH. If voice (AMR voice data) does not utilize the HS channel, then voice data is transmitted from base station 1 to mobile station 4 on DPCH. If the handover state is attained during this simultaneous communication [see (C) of FIG. 15], the same voice data is sent from both base stations 1 and 2 on DPCH but packets are sent from only one base station, namely base station 1 with which the mobile station was communicating up to this point, or base station 4 with which the mobile station communicates after handover. For this reason, it has been proposed to obtain the weighting coefficient w by the following equation:w=arg max wH[αH1HH1+(1−α)H2HH2]w  (4)instead of Equation (3) in handover during communication on HS-PDSCH. In the equation above, the value of the coefficient α is selected to be between 0.5 and 1.0. If packet transmission via HS-PDSCH is from base station 1 and a is made 1.0 to stress the receive signal from base station 1, then the data (packet) on HS-PDSCH can be received in the best form. With regard to the voice data on DPCH, however, the voice data from base station 2 cannot be used, diversity gain is not obtained and voice quality declines. On the other hand, if a is made 0.5 and the receive signals from both base stations 1 and 2 are handled equally, diversity gain will be obtained with regard to the voice data and voice quality will be improved, though the receive quality of the data (packet) on HS-PDSCH will decline. In other words, a is in a trade-off relationship between HS-PDSCH and DPCH.
Accordingly, a method illustrated in FIG. 16 has been proposed (see Reference 3).
Reference 3: R1-02-1374 [TSG-RAN Working Group 1 meeting #29, Shanghai China, Nov. 5-8, 2002, Agenda item: 6.2-HSDPA Applicability of TX diversity (closed loop) modes, Title: Further Summation Results on Fast Switching proposal]
According to this method (referred to as a “Fast Switching Operation”), stress is placed upon the signal from base station 1 and α is made 1.0 during an HS service, and α is made 0.5 in order to achieve diversity gain during a non-HS service.
However, since packet data is transmitted via a shared channel even during a HS (high-speed transmission) service, the data will not necessarily be transmitted to the same mobile station every subframe.
Of course, even during an HS service (a state in which HS-SCCH is being monitored to prepare for reception on HS-PDSCH), data to be transmitted from the base station to this mobile station runs out and there are also instances where there is no transmission of packet data to this mobile station via the HS-PDSCH for some time.
Further, it is necessary to take into consideration the interval during which α is made 1.0 by the Fast Switching Operation, namely the interval during which weighting is applied to the signal received from the base station that transmits HS-PDSCH, as well as processing delay time needed to execute decision processing for applying weighting. FIG. 17 is a diagram for describing processing delay time. FIG. 17 illustrates two subframes (=six slots) of one frame (3 slots×5=15 slots). In a case where it has been specified to transmit a packet to a certain mobile station by the third slot SL3 of subframe SF1 of HS-SCCH in the handover state, a delay time TD is required until FBI prevailing at the time of α=1.0 is transmitted to the base station. That is, if t1 represents the time it takes for the mobile station to perform modulation and identify whether there is a packet addressed to it following receipt of HS-SCCH in subframe units, t2 represents the time needed to detect the phase difference of CPICH and calculate the weighting coefficient w from the phase difference, and t3 represents the time until FBI is decided from the weighting coefficient w and a transmission is made to the base station, then a delay time of TD=t1+t2+t3 will be required. As a consequence, at the start of the slot SL3′ in which the base station transmits a packet, the transmitter cannot apply weighting conforming to the FBI prevailing at the time of α=1.0 and the performance of the Fast Switching Operation cannot manifest itself. Thus, in a case where data (a packet) transmitted to the mobile station itself is sent in bursts, i.e., in a case where data is transmitted intermittently by the scheduling of the base station, a problem which arises is that the performance of the Fast Switching Operation cannot manifest itself effectively.